How should I be running my DB migrations in a AWS Serverless application? In a traditional NodeJS app, I usually have npm start run sequelize db:migrate first. But with Lambda how should I do that?
My DB will be in a private subnet. Was wondering if CodeBuild will be able to do it? Was also considering to have a Lambda function run migration ... not sure if its recommended tho.
There are a number of ways to achieve this. You actually are kind of on the right track with CodeBuild, at least there shouldn't be anything wrong with taking that approach.
Since your DB is in a private subnet, you will need to configure CodeBuild to access your VPC. Once you have that configured, it's a simple matter of allowing access from the CodeBuild security group to your database.
You might want to setup this whole thing as a CodePipeline. You might even set it up with multiple buildspec files for different runs of CodeBuild. That way you can have a CodePipeline that looks like:
Source → CodeBuild (test) → Approval → CodeBuild (migrations) → Lambda
Theoretically, you could also create a Lambda function that does the migration, and trigger that as needed. If the migrations take a long time, you could also use AWS Batch to run them. But using CodeBuild as part of a deployment pipeline makes a lot of sense.